An application of Pascal's Law: a change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container.

Usually whist dancing, but not exclusively so, a person, usually female, while rub her rump on the crotch of a guy. During this exchange the junk of the man converts to the same properties as an enclosed incompressible fluid and if timed right can lead to the male's orgasm. A combination of the 'bumping uglies' and 'grinding.'